The mysidNeomysis mercediswas examined as a test organism for use in acute toxicity tests at intermediate salinities characteristic of estuarine waters. Several sensitive invertebrate species are available for marine assessments (mysids) and freshwater tests (cladocerans), but few are available for estuarine toxicity tests. Observations in the laboratory indicate thatNeomysis mercediscan be reared successfully at a temperature of 17°C, a salinity of 2‰, and a population density less than 5/L. Brine shrimp naupliiArtemia salina, algae, and commercial foods were used to sustain mysid cultures.Neomysis mercedisis viviparous and can complete its life cycle in 3–4 months.Neomysis mercedisis as sensitive as or more sensitive to toxicants than the marine mysid Mysidopsis bahia and the freshwater cladoceransDaphnia magna,Ceriodaphnia dubia, andSimocephalus serrulatus. The mean 96-h LC50 values (concentrations lethal to half the test animals) forN. mercedis, in increasing order, were 0.20 μg/L for methyl parathion, 2.2 μg/L for malathion, 14 μg/L for carbofuran, 150 μg/L for copper sulfate, 280 μg/L for thiobencarb, and 1,600 μg/L for molinate. Neonates (5 d postrelease) were generally more sensitive than older juveniles. Coefficients of variation (100·SD/mean) of LC50 values varied from 21 to 35%.